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Bureau of Mines Information Circular/1985 



A Low-Cost FSK Modem Network 
for Polled Communication Systems 



By Richard A. Watson 




UNITED STATES DEPARTMENT OF THE INTERIOR 



1751 

'Wines 75TH ax^"^ 



i 



Information Circular 9021 



A Low-Cost FSK Modem Network 
for Polled Communication Systems 



By Richard A. Watson 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Model, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 




Libtaty of Congress Cataloging in Publication Data: 



no. ^0^1 



Watson, Richard A., 1948- 

A low-cost FSK modem network for polled communication systems. 

(Information circular ; 9021) 

Includes bibliographical references, 

Supt. of Docs, no.: I 28.27:9021. 

1. Mine communication systems. 2. Modems. I. Title. II. Title: 
Low-cost F.S.K. modem network for polled communication systems. III. 
Series: Information circular (United States. Bureau of Mines) ; 9021. 



-T!<r29-5rU4— [TN3441 622.s [622', 81 



84-600286 



V^^" 



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< 



CONTENTS 

Page 

Abstract I 

Introduction 2 

Purpose and requirements 2 

General description 3 

Theory of operation 4 

Basic overview 5 

LSM operation 6 

RCCM operation 9 

Other applications 11 

Conclusion. 12 

Appendix A. — Frequency selection and filter design 13 

Appendix B. — Modulation design and calibration 18 

ILLUSTRATIONS 

1 . Modem frequency spectrum 4 

2 . Network diagram 4 

3 . LSM block diagram • 3 

4 . RCCM block diagram 6 

5 . Waveform timing diagram 6 

6. LSM schematic 7 

7. Line splitter schematic. 9 

8. RCCM schematic 10 

A-1. Chebyshev bandpass filter 14 

A-2. Filter response 16 

A-3. RCCM filtered response 17 

B-1. Demodular circuit 18 

B-2. Modulator circuit 19 

B-3. RCCM calibrator schematic 21 

B-4. LSM printed circuit layout 22 

B-5. Line splitter printed circuit layout 23 

B-6. RCCM printed circuit layout 23 





UNIT OF MEASURE 


ABBREVIATIONS 


USED 


IN THIS 


REPORT 


Bd 


baud (bits per second) 




Mfi 


megohm 


dB 


decibel 








ms 


millisecond 


dB/mi 


decibel per mile 






yF 


microfarad 


ft 


foot 








ys 


microsecond 


h 


hour 








a 


ohm 


Hz 


hertz 








pF 


picofarad 


kHz 


kilohertz 








V 


volt 


kn 


kilohm 








w 


watt 


MHz 


megahertz 













A LOW-COST FSK MODEM NETWORK FOR POLLED COMMUNICATION SYSTEMS 

By Richard A. Watson' 



ABSTRACT 

A frequency-shift keying (FSK) modulator-demodulator (modem) network 
has been devised for the Bureau of Mines mine-monitoring systems. This 
network permits an unlimited number of remote stations to communicate 
with one central station over the same pair of wires. The network oper- 
ates at a speed of 4,800 Bd at distances up to 5 miles. 

This report describes the development and operation of the modems. 
The theory of operation, schematic diagrams, and printed circuit board 
layouts are provided. Although the modems were built for a specific ap- 
plication, a more general use is discussed whereby the network would op- 
erate with a typical serial channel format. 



'Electrical engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



The mining industry is finding that the 
use of monitoring systems can increase 
safety and production. Remote monitoring 
gives management real-time information 
about the underground environment and 
equipment status. It also provides the 
ability to remotely control equipment 
such as belts and pumps. With this in- 
formation at hand, hazardous conditions 
and equipment failures are quickly recog- 
nized and appropriate actions are more 
immediate. 

As the benefits from these monitoring 
systems become documented, their growth 
can be expected to increase; with this, 
the distance and number of monitoring and 
control functions will increase. Two im- 
portant criteria must be considered: 
system speed and data security. 



The Bureau has researched monitoring 
systems for the purpose of establishing 
performance guidelines. Out of this re- 
search, a special FSK modem has been de- 
vised for use in the Bureau's monitoring 
systems. This modem permits operation 
over standard telephone wiring at dis- 
tances up to 5 miles, with no limit on 
the number of remote outstations. It 
maintains the high speed of the baseband 
link used for shorter distances and pro- 
vides for lower error rates. 

Although this modem was developed for a 
special application that may be limited, 
another application has been considered. 
It entails modifying the modem to operate 
with a typical asynchronous 11-bit serial 
channel. This modification is being 
tested and is discussed in some detail in 
the section "Other Applications." 



PURPOSE AND REQUIREMENTS 



The existing communication link used in 
the Bureau's monitoring projects consists 
of baseband signals transmitted over a 
special heavy conductor cable. Two of 
these cables are used in the intrinsi- 
cally safe mine-monitoring system (ISMMS) 
installed at the Lucerne No. 8 Mine, 
Clarksburg, PA. 2 Each cable is used to 
communicate with and power environmental 
transducers. 

When expansion of the belt-monitoring 
system was planned, another method of 
communication to the underground was de- 
sired. Modems were considered, because 
they would employ available pairs of wir- 
ing in the underground telephone network. 
This would eliminate the costs and ef- 
forts of installing more cable. The spe- 
cial cable was not required since belt 
monitoring was not included in intrinsic 
safety operation. 

^Watson, R. A. An Intrinsically Safe 
Environmental Monitoring System for Coal 
Mines. Paper in Proceedings of the Sixth 
WVU Conference on Coal Mine Electrotech- 
nology (Morgantown, WV, July 28-30, 
1982). WV Univ., Morgantown, WV, 1982, 
pp. 345-360. 



There are about 15 belt drive control- 
lers in the Lucerae No. 8 Mine. With the 
addition of several pump-monitoring sta- 
tions, it was obvious that the number of 
remote modems should not be limited. 
Each remote modem must connect with the 
local modem at the central station on the 
surface. The modems should be transpar- 
ent to the monitoring system, receiving 
and transmitting compatible signals at 
each end. 

The picture that was rapidly developing 
was of a system similiar to that of local 
networks. With local networks, many us- 
ers have the ability to share the re- 
sources of a centralized computer via a 
common communication link. Some networks 
use carrier sense multiple access with 
collision detection (CSMA/CD) and rely 
upon remotes' transmitting on a random 
basis, with provisions for detecting col- 
lisions. Other networks use a token 
passing scheme where information is sent 
around a loop and each station takes or 
injects what it needs. These, as well as 
other types of networks, are complicated, 
costly, and not directly compatible with 
the polling scheme used in the Bureau's 
monitoring systems. 



In order to have an unlimited number of 
remote modems connected onto the same 
line, each remote must be seen as a high 
impedance. The receiving section should 
be high-impedance-buffered, and the 
transmitting section should be discon- 
nected when not required to transmit. 
The line is then terminated, at the re- 
spective ends, with a resistance equal to 
the characteristic impedance of the 
line. 



Both local and remote modems should be 
guarded against electrical noise, spikes, 
and surges on the line. The degree of 
protection should be high, as mine elec- 
trical disturbances are known to be con- 
siderable. Additionally, the local modem 
connection to the monitoring system 
should be optically isolated from lines 
that enter the underground. 



GENERAL DESCRIPTION 



An FSK modulator converts binary digi- 
tal values, representing logical "ones" 
(1) and "zeros" (0), into two frequen- 
cies. They are known as the mark and 
space frequencies, respectively. The de- 
modulator reconverts these frequencies 
into digital values at the receiving end. 
Exar Integrated Systems, Inc.,3 manufac- 
tures integrated circuits (IC's) that do 
the conversions with a small number of 
external components. Two of these IC's 
were selected because of their low cost, 
ease of use, and well-documented applica- 
tion notes. They are the XR2206 modula- 
tor and XR2211 demodulator. The pair can 
accommodate signals over a 60-dB dynamic 
range of frequencies as high as 300 kHz. 

The modulator and demodulator IC's are 
used in both the local and remote modems. 
In the local modem, the carrier is always 
on. A crystal-controlled clock synchro- 
nizes timing with the transmitted message 
and "opens" the receiver when the receiv- 
ed message is due. This modem will be 
referred to as the local synchronous 
modem (LSM) . In the remote modem, the 
carrier is turned on only when activated 
by the station responding through the 
modem. This modem will be referred to as 
the remote carrier-controlled modem 
(RCCM). 

Although the mark and space frequencies 
of the modem IC's cover a wide range, the 
choice is not completely arbitrary. 

^Reference to specific manufacturer or 
trade names is for identification only 
and does not imply endorsement by the 
Bureau of Mines . 



Selection of these frequencies is based 
upon the channel bandwidth, line attenua- 
tion, and the system baud rate. Addi- 
tionally, if a full duplex channel (simu- 
ltaneous transmission in both directions 
at the same time) is selected, the mark 
and space frequencies of the two groups 
must be separated by enough distance to 
prevent interference. This separation is 
determined based on the filtering re- 
quirements of each group. 

Figure 1 is an illustration of the two 
groups of frequencies. The mark and space 
frequencies transmitted from the LSM are 
fl and f2, respectively. Those received 
from the RCCM are fj, and f/^. The selec- 
tion of these frequencies and design sol- 
utions are covered in the appendixes; the 
frequencies are given here as 12.6, 16.2, 
23.4, and 27.0 kHz, respectively. The 
selection avoided too high a frequency 
where line attenuation is greater and 
line balancing and matching become 
important. 

Separate pairs of wires are used for 
the transmit and the receive frequencies. 
A line hybrid could combine the two 
groups onto the same line but would be 
incompatible with the switching technique 
of the transmitter. A hybrid is used to 
balance a line and prevent the transmit- 
ted output from feeding back into its re- 
ceiver input. The balance is based on a 
fixed line impedance as seen by both the 
output and input and is used where modems 
are connected one to one. When many re- 
mote modems are connected to the same 
line and are being switched on and off, 
the line impedance is not constant. 




f 



'2 '3 

FREQUENCY SPECTRUM 

FIGURE 1. - Modem frequency spectrum. (H(jw) = frequency response of a network function; H = 
frequency response in the bandpass.) 



With separate transmit and receive 
lines, it is not neccessary that the two 
groups of frequencies be separated, so 
long as line and circuit crosstalk are 
low. However, given the rugged environ- 
ment of the mining industry, the line 
conditions cannot be guaranteed. Bad 



splices and line leakage could be expect- 
ed with time. Because of these consider- 
ations, the groups were separated and 
filters were included at each receiver to 
guarantee at least 24-dB attenuation to 
out-of-band signals. 



THEORY OF OPERATION 



Operation of the modems requires one 
LSM modem connected to at least one RCCM 
modem, a 4,800-Bd serial computer channel 
(monitoring system) , and a remote trans- 
ponder. ^ The monitoring system communi- 
cates two 11-bit words (back to back) to 
the transponder through the LSM and RCCM, 
as shown in figure 2. The transponder 
then enables the carrier of the RCCM and 
communicates its reply of two 11-bit 
words to the monitoring system through 
the RCCM and LSM. There are no limits to 
the number of RCCM's connected to the LSM 
as each are seen as a high impedance. 
The number of transponders connected to 
each RCCM can also be quite large and is 

■^The transponder used in the Bureau's 
monitoring system is a special device. 
Upon recognition of a given address and 
command, it issues a synchronizing pulse 
followed, 1 1 bit times later, with two 
data words. It is essential to the oper- 
ation of the LSM and RCCM in the original 
development. 



limited only by the supply current, line 
length, and definition capabilities of 
the monitoring system. 



LSM 



Terminators 



4-wlre cable 
2 transmit 
2 receive 



RCCM 



^, 



© ® 



RCCM 



^ 



<::> 



I 



4-wire cable 

+V 

-V 
Transmit 
Receive 



MS 



LSM 
RCCM 



KEY 
Monitoring system or any 
serial computer channel 
Local synchronous modem 
Remote carrier-controlled 
modem 
Transponder 



FIGURE 2. - Network diagram. 



BASIC OVERVIEW 

Figure 3 is a block diagram of the LSM. 
Communication between the monitoring sys- 
tem and the LSM is by way of RS232 speci- 
fied connections. Optical couplers pro- 
vide electrical isolation of the signals. 
The transmitted signal is passed to the 
modulator and the synchronous circuit. 
Mark and space frequencies from the modu- 
lator (f ] and £2) are coupled to the line 
through a transformer and line protection 
circuit. Precise timing of the synchro- 
nous circuit opens the channel gate when 
the transponder's reply is due. 

The responding mark and space fre- 
quencies (f3 and f^) from the RCCM are 
coupled to the demodulator through the 
transformer and line protection circuit 
and a bandpass filter. A reference 
oscillator (at frequency f ^) keeps the 
demodulator chatter down should a 
transponder fail to respond in the re- 
quired timeframe. The demodulated 
signals signify reception to the 
synchronous circuit and are then passed 



through the gate to the monitoring 
system. 

Figure 4 is a block diagram of the 
RCCM. The mark and space frequencies 
from the LSM (f , and f2) are coupled to 
the demodulator through a transformer and 
line protection circuit, a high-impedance 
buffer, and a bandpass filter. Demod- 
ulated signals are then passed directly 
to the transponders. The original RCCM, 
as given in the section "RCCM Operation," 
relies on the transponders to issue a 
synchronizing pulse that triggers the 
carrier and to delay the data to be 
transmitted. The updated version, 
discussed in the section "Other Applic- 
ations," provides the trigger and delay 
directly on the RCCM. Although this 
increases the parts count and complexity 
of the RCCM, it provides for a more 
general use. 

Modulated data from the RCCM (fj and 
f ^) are passed to the line through a 
solid-state switch and transformer and 
line protection circuit. The switch pro- 
vides greater than 50 dB of isolation in 



Mine 

monitoring 

system 

CPU 



TXD 



COMM 



RXD 



Optical 
coupler 



* -> 



Synchronous 
circuit 



Optical 
coupler 



Modulator 




Reference 
oscillator 



Gate 



Dennodulator 



Trans- 
former 
and 
line 
protection 



Bandpass 
filter 



Line 
out 



FIGURE 3. - LSM block diagram. 



Line in- 



Line out- 



Transformer 

and line 

protection 




High- 
impedance 
buffer 


^- 


Bandpass 
filter 




Demodulator 


— ►D 


^ 
























Carrier 
control 














* 










^ 




.^ 


Switch 


^ 


Modulator 


^ 




^ Delay ^ 








* 











Data in 



FIGURE 4. - RCCM block diagram. 



the OFF state and less than 0.3-dB loss 
in the ON state. Thus, the line is not 
loaded by the output impedance of the 
modulator when not in the transmit mode. 

LSM OPERATION 

This discussion deals with the original 
LSM. Refer to the timing diagram of fig- 
ure 5 and the schematic diagram of figure 
6. The updated version under test is 
discussed in the section "Other Applica- 
tions." The appendixes can be consulted 
for specifics on the operation of the 
modulator, demodulator, and filter. 

The transmitted signal enters the LSM 
from the monitoring system at connector 
Jl pin 3 or from a portable tester at J2 
pin 2. Waveform 1 of figure 5 shows the 
transmitted signal as seen at test point 
TP2. This signal enters the modulator, 
IC2, at pin 9. Potentiometers R55 and 
R57 adjust the frequencies f, and f2. 
R61 sets the output amplitude to dB as 
measured at TP3. Diodes D3 and D4 and 
resistor R9 provide line protection for 
surges below the threshold of the gas 
discharge tube on the secondary side of 
transformer T2. 

The transmitted signal also enters the 
synchronous circuit at IC7 pin 6. Pin 5 
is high at bit time (beginning of 



I I I I I I I I l | I I I I I I I I l| I I I I I I I I l | I I I I I I I I l | I I I I I I I M | I M I I I I I I 



Transmit 1 1 Transmit 






Receive 
I 



Receive 
2 



I 1 1 1 1 1 1 I 



ill,,, 



20 30 

TIME DIAGRAM, 



40 
bit time 



50 



60 



FIGURE 5. - Waveform timing diagram. 



I 




u 



LU 

Z) 

o 



transmitted message) and can be measured 
at TP7. Waveform 4 is the signal at TP7 
and is known as the channel-open pulse. 
With coincidence of the transmitted sig- 
nal, a flip-flop of IC5 is set and pin 2 
goes low as shown in waveform 2. 

An oscillator circuit, running at 
2.4576 MHz, is used as a clock input to 
dividers IC3, IC4, and IC,3. With a low 
on pin 11, IC3 begins to count clock pul- 
ses until bit time 16 occurs. At this 
time, a high at pin 3, waveform 3, sets 
two flip-flops of ICg at pins 6 and 8. 
ICg pin 2 goes low and thus opens the 
channel for reception and blocks the 
transmitted message from setting IC5 with 
data pulses. ICg pin 12 also goes low, 
as shown in waveform 5, and begins the 
count of the time-out timer, IC13. 
Shortly after bit time 16, IC7 pin 3 goes 
high and resets ICg at pin 4. Meanwhile, 
IC , 3 continues counting for 16 more bit 
times and then produces a time-out pulse 
at pin 3. This pulse is shown in wave- 
form 6 and is the reset to ICg pin 10. 

At bit time 16, the channel is open 
(waveform 4) and the RCCM should be re- 
ceiving the second word of the message 
shown in waveform 1. At the completion 
of this message, the reply from the RCCM 
is expected. 

The reply from the RCCM can be monitor- 
ed at TP4. Dl, D2, and RIO provide low- 
level protection for the received lines. 
After passing through the buffer and 
bandpass filter of IC,2j the mark and 
space frequencies are delivered to the 
demodulator, IC^. R27 adjusts the center 
frequency between f3 and f^, D5 indi- 
cates a carrier level sufficient for 
reception. 

ICj4 is another modulator set to oper- 
ate at the receive space frequency, f/^. 
R38 adjusts the frequency, and R39 is 
adjusted to provide about a -50-dB level 
as measured at TP9. This modulator 
serves as a reference oscillator to the 
phase-locked loop of the demodulator. It 
prevents chatter from appearing at the 
demodulated data output, pin 7, when no 
response is received and the channel is 
open. Normally, pin 6, tied to pin 7, 
holds the data output off when no carrier 
is present at the input, pin 2. However, 



the tight bandwidth of the filter atten- 
uates out-of-band noise and increases the 
probability of momentary phase lock on 
in-band noise and, thus, data chatter. 

The demodulated data output is measured 
at TP6 and shown as waveform 7. The 
pulse at bit time 10.5 is intermittent 
and occurs about half the time. It is a 
result of the carrier's being just turned 
on at the remote RCCM. Although the RCCM 
turns on at the same frequency as the re- 
ference oscillator (f4), phase differ- 
ences cause the momentary pulse. This 
pulse is the primary reason for the 
synchronized channel-open pulse. Typical 
modems, connected one to one, usually 
have this pulse only during connect time. 
It is commonly known, and software proto- 
cols account for its presence. As there 
was no control over the software for this 
network, the solution was provided in 
hardware. 

The demodulated data at TP6 are coupled 
to the monitoring system through Jl pin 
2, or to the portable tester at J2 pin 1, 
and are also fed to the synchronous cir- 
cuit. It is logically OR'd with the 
time-out pulse and logically AND'd with 
the Inverse channel-open pulse at ICg pin 
1. The resulting pulse sets IC5 flip- 
flop at pin 8. A low at IC5 pin 12 en- 
ables IC4 to count for 24 bit times. At 
the end of that time, IC7 pin 10 goes 
high and resets the counter and channel- 
open pulse. Waveform 8 is actually the 
Inverse of the 24-bit time at pin 12 of 
IC5, and shows the OR'lng of waveforms 6 
and 7 to set the time for waveform 4, the 
channel-open pulse, to end. The synchro- 
nous circuit is now reset and awaits an- 
other transmission. 

For implementation at the Lucerne No. 8 
Mine, the transmitted signals needed to 
be split into two directions, one for the 
south mains line and one for the north. 
To accomplish this, the circuit of figure 
7 was connected between the LSM and the 
RCCM's. The gas discharge tubes were 
left off the secondaries of transformers 
Tl and T2 of the LSM. Instead, the 
transformers were connected to Tl and T2 
of the line splitter, and here the gas 
tubes were placed on the secondaries. 
The transmitted signal is equally split, 



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R8 

— nAAA- 



R7 



< TB2-I 



< TB2-2 

< TB2-9 



North TXD 



South TXD 



< TB2-8 

< TB2-5 Shield 



< TB2-6 



< TB2-7 

< TB2-4 



South RXD 



< TB2-3 



North RXD 



Material list 

Resistors R1-R8: 10ii,2W 
Gas discharge tubes G1-G4: Tl I 339 

Transformers T1,T2: TRIAD SP-69 
Terminal board TB1: RDI3PCV-05 
Terminal board TB2: RDI3PCV-09 

FIGURE 7. - Line splitter schematic. 



and the 600-fi output impedance of the LSM 
modulator is transformed to 150 fi. This 
impedance more accurately reflects that 
seen in underground wiring. Since all 
RCCM's reflect a high impedance, a 150-fi 
resistor is connected at the end of the 
transmit lines in both the north and 
south mains. 

RCCM OPERATION 

The following discussion deals with the 
operation of the RCCM. Refer to the 
schematic of figure 8 and waveforms of 
figure 5. 

The mark and space frequencies from the 
LSM are received through transformer Tl. 
Surge protection is provided by the same 
methods as for the LSM. The received 
message is delivered to the demodulator, 
IC,,. through the buffer and bandpass fil- 
ter of IC5. R24 adjusts the center fre- 
quency between f j and f2. D6 indicates a 



received carrier of sufficient level for 
reception. The demodulated data are 
passed to the transponder through Jl pin 
2 and can be measured at TP3 as that of 
waveform 1. 

The reply from the transponder can be 
measured at TP4 and is the inverse of 
waveform 7. The pulse at bit time 10.5 
triggers IC4 at pin 2. This trigger 
causes pin 3 to go high and results in 
DIO going off and IC3 going on. IC3 is a 
solid-state field effect transistor (FET) 
switch with four stages in parallel. The 
total ON resistance is about 10 Q.. At 
this moment, the carrier is on and t^ is 
being transmitted back, to the LSM through 
T2. IC3 will remain on until IC4 times 
out. This time is controlled by R26 and 
C16 and is approximately 9 ms. This is 
enough time, including component toler- 
ances, to leave the carrier on for com- 
plete transmission of the transponder's 
reply. 



10 




u 
u 



a: 

O 



11 



The reply from the transponder is also 
fed to the modulator IC2 at pin 9. R32 
and R34 set the f^ and f^ frequencies 
while R30 sets the output level as mea- 
sured at TP6. When IC4 times out, DIO 



goes back on and IC3 goes off. With IC3 
off, the output line becomes high imped- 
anced and the next RCCM that transmits is 
not loaded by the output impedance of 
IC2, reflected through T2. 



OTHER APPLICATIONS 



As mentioned previously, the circuits 
presented here are those of the original 
development, which rely on the synchro- 
nizing pulse from the transponder for 
proper operation. These transponders are 
located in the same enclosure as the 
RCCM, and the synchronizing pulse has a 
short distance to travel. If the trans- 
ponders were to be located at a further 
point down the line, the pulse (about 20 
s long) would be dampened out by cable 
capacitance. 

Another application has been encoun- 
tered that could make use of the modem 
network. But, in this network, the 
transponders would be located at dis- 
tances up to 8,000 ft from the RCCM. 
Operation of the circuits could not rely 
on the synchronizing pulse. The RCCM 
must gate the carrier on at the beginning 
of the reply and then delay the reply 
until the line stabilizes. This requires 
the addition of a clock circuit and data 
shift registers but provides operation 
with a typical asynchronous serial chan- 
nel format. 

The main concept of the network still 
remains the same. The LSM receives two 
data words from the monitoring system (or 
any serial computer channel) and returns 
two data words in reply. The words must 
be back to back at 4,800 Bd, with a posi- 
tive start bit into the LSM and a nega- 
tive into the RCCM. 

A breadboard version of the modified 
RCCM and LSM has shown successful re- 
sults. Although the modification is dis- 
cussed here, full documentation cannot be 
available until field testing is com- 
plete. For more information concerning 
this modification, consult the author. 

Refer to the RCCM schematic (fig. 8) 
for a brief discussion of how the RCCM is 



modified for testing of the breadboard 
version. The reply from the transponder 
enters the circuit through diode D9. The 
leading edge of the reply toggles a flip- 
flop that gates the carrier on, via IC3 , 
and enables clocking of a 4-bit shift 
register and a counter. A clock circuit, 
identical to that of the LSM, is divided 
down by the Q9 output of the counter and 
fed to the clock input of the shift reg- 
ister. The reply is then clocked through 
the registers and arrives at IC2 , 3.5 bit 
times later. A Q15 output of the counter 
resets the flip-flop and thus resets the 
counter and the shift register, and turns 
the carrier off. 

The modifications to the LSM can be 
performed more simply. Refer to the LSM 
schematic (fig. 6) and the timing diagram 
(fig. 5). TP6 is the demodulated data 
received from the RCCM. In the modified 
version, the intermittent pulse at bit 
time 10.5 will now occur at bit time 21.5 
(the moment the transponder begin 's its 
reply and turns on the carrier) , and the 
reply will arrive at bit time 25 (3.5-bit 
delay from the shift register of the 
RCCM). The LSM can be modified to open 
the channel at bit time 24, thereby miss- 
ing the intermittent pulse and enabling 
the reply. The channel can then be 
closed 24 bit times later using the ex- 
isting 24-bit timer. 

To open the channel, the pulse of wave- 
form 3 is moved to bit time 24 by AND'ing 
the Q13 and Q14 outputs of IC3. The re- 
sultant pulse can then be fed to IC7 pin 
2, ICg pin 6, and IC5 pin 8. The output 
of IC7 pin 11 should be disconnected, and 
the time-out circuit of IC13 is no longer 
needed. 



12 



CONCLUSION 



The modem network described in this re- 
port is an inexpensive modulation tech- 
nique for communication systems operating 
in a polling mode. As such, it can com- 
municate to an unlimited number of re- 
motes at a relatively high baud rate. 

The circuitry presented can be easily 
modified to accommodate different baud 
rates and word lengths. The procedures 
here and in the indicated references can 
be used to calculate component values for 
the modulator and demodulator circuits. 
The filter circuit is more complicated 
and may not be necessary for many applic- 
ations. If independent lines are used 
for transmit and receive, and if circuit 
crosstalk and noise are reasonably low, 
then the demodulator will operate with a 
reasonably low bit-error rate. 

With an output power level of dBm, 
the dynamic range of signal detection is 



50 dB. Laboratory tests show that with a 
signal-to-noise ratio (SNR) of 20 dB , the 
error performance is on the order of 
10"^. Preliminary field testing with 
five remote modems distributed along the 
north and south sections of the mine 
revealed line losses averaging 6 dB/mi 
and error rates about 10"^. If the 6-dB 
loss per mile is typical, distances be- 
yond 5 miles would degrade the perform- 
ance and result in error rates greater 
than 10" 5. 

The effects of impulse noise induced 
into the communication lines from load 
switching or other sources could also re- 
duce the SNR enough to increase error 
rates. As in any modulation scheme, the 
circuit is never assumed free from error 
and some type of information protocol 
should be considered. 



13 



APPENDIX A.— FREQUENCY SELECTION AND FILTER DESIGN 



FSK signals are commonly used to trans- 
mit digital information over telephone 
lines. In this type of modulation, the 
carrier signal is shifted between two 
discrete frequencies to encode the binary 
data. These two frequencies are produced 
by a function generator that is keyed 
with the binary data. This generator is 
then referred to as the modulator. A de- 
modulator, such as those of phase-locked 
loop (PLL) circuits, is then centered be- 
tween the two frequencies and reproduces 
the binary signal as the frequencies 
shift back and forth. 

The specifications of the function gen- 
erators and PLL circuits include guide- 
lines for selecting the two discrete fre- 
quencies. These guidelines are based 
upon the baud rate and carrier frequency, 
which are determined by the available 
bandwidth of the communication channel. 

The typical communication channel for a 
monitoring system utilizes telephone wir- 
ing. Twisted-pair conductors of No. 16 
or No. 18 gauge are very common sources 
of mine wiring. In an environment free 
from electromagnetic inteference, this 
type of wiring could carry 4,800 Bd com- 
munications over a 50-kHz carrier for 
many miles. But documented evidence^ 
shows that electromagnetic interference 
in average mines is significant enough to 
limit this kind of coimnunication to a 
mile or two. 

Shielding twisted-pair wiring improves 
the immunity to electromagnetic interfer- 
ence but increases the overall cable ca- 
pacitance. This increase may be more 
detrimental to a 50-kHz carrier in the 
way of line loss than would the interfer- 
ence. The environment then must be con- 
sidered when selecting a cable, the baud 
rate, and the carrier frequency for a 
communication channel. In designing sys- 
tems for general purposes, severe or 
worst cases should be assumed or a 

^Bredeson, J. G., J. L. Kohler, and H. 
Singh. Data Security for In-Mine Trans- 
mission. Final Report — Part I (contract 
J0308024, Univ. OK). BuMines OFR 76-81, 
1981, 99 pp.; NTIS PB 81-221988. 



variety of communication channels should 
be designed. 

The function generator and PLL circuits 
used in the mine monitoring system modem 
network are the IC's XR2206 and XR2211, 
respectively. The specific design using 
these IC's is covered in appendix B. 
However, several operational characteris- 
tics of the PLL must be considered here 
in first selecting the frequencies of op- 
eration and filtering requirements. 

Application note AN-Ol^ gives the fol- 
lowing guidelines for calculating non- 
standard frequencies: 

• The lower frequency, f,, must be at 
least 55% of the upper frequency, f2 
(less than a 2:1 ratio). 

• Select fi and f.2 higher than the 
baud rate, fp, for minimum pulse width 
jitter. 

• For maximum fj and f2 spacing (where 
the ratio is close to 2:1), use the rela- 
tionship (f2 - fi)/fp > 83%. 

• For narrower f^ and f2 spacing, use 
the relationship (f2 - f])/fp > 67%. 

Athough "narrow spacing" is not too 
well defined, some worked-out examples of 
the application note for standard modems 
result in the following frequency ratios 
and the relationships used: 



Baud 
rate 


Frequency 
ratio 


Relationship, 

% 


300 

300 

1,200 


1.10:1 
1.18:1 
1.82:1 


67 
67 
83 



The communication channel at the Lu- 
cerne No. 8 Mine would make use of two 
available pairs in a multipair telephone 
cable. Most of the pairs in the bundle 
are used for telephone communications to 
the underground section. For this rea- 
son, the frequencies selected for modem 
operation should be above the baseband of 
voice signals. Since the pairs available 
for use are direct wires (no telephone 
loading coils or other circuitry), the 

^Exar Integrated Systems, Inc. (Sunny- 
vale, CA) . Application Data Book. June 
1981 , 80 pp. 



14 



upper limit on the frequencies could be 
based on propagation losses. 

In the frequency spectrum (see figure 
1)-^ for the available communication chan- 
nel, f ] and the general shape along the 
bandpass curve to its left should be 
above the voice band, and f^ should be as 
low as possible to minimize line losses 
and standing waves that might occur as a 
result of mismatching: preferably below 
50 kHz. 

From the guidelines and worked-out ex- 
amples given above, the consideration 
that the carrier frequencies may lie 
around the 20-kHz range, and the 4,800-Bd 
rate of the monitoring system, a rela- 
tionship of 75% would work well. 
Therefore, 

(f2 - f ,)/fp = 0.75. (A-1) 

Then the frequency spacing, df , is 



nine resistors. The general layout of 
the circuit is that of figure A-1. This 
circuit is used with the demodulators for 
group 1 (fi and £2) and group 2 (f^ and 
f4). Using the monograph in the applica- 
tion note, a bandstop-to-bandpass ratio 
of 2:1 for this filter would yield a min- 
imum bandstop attenuation of 24 dB. This 
will be sufficient attenuation for out- 
of-band noise and for crosstalk inter- 
ference between the two groups of 
frequencies. 

This application note also recommends 
that the filter bandpass be twice the 
mark-space separation. Then the band- 
pass, bp, from equation A-2 is 

bp = 2 df = 2 * 3,600 Hz 

= 7,200 Hz, (A-3) 

and the bandstop, bs , is 



df = f2 - f 1 = 0.75 fp = 0.75 * 4,800 



bs = 2 bp = 2 * 7,200 Hz 



= 3,600 Hz. 



(A-2) 



= 14.4 kHz. 



(A-4) 



Application note AN-03^ suggests the 
use of a three pole-pair Chebyshev filter 
with a bandpass ripple of 1 dB. The jus- 
tification is that the filter can be con- 
structed with just three operational am- 
plifiers (one IC), six capacitors, and 

o 

-"Figure numbers without an A- prefix 
refer to figures in the main text. 
'*Work cited in footnote 2. 



One further consideration before the 
frequencies can be determined is that of 
harmonic interference between the two 
groups. The function generators used in 
the modulator have a sine wave output 
with low distortion. When the frequen- 
cies are selected, the third harmonic of 
frequencies in the first group must not 
lie in the spectrum of the second group. 
More precisely, the third harmonic of the 



R1A 

o-wvMHh^ 



R2A 




V <7 



V V 



FIGURE A-1. • Chebyshev bandpass filter. 



15 



lowest frequency in the bandpass of the 
first group is greater than the highest 
frequency in the bands top of the second 
group. Translated into equations, it is 

3(fcl2 - bp/2) > fc34 + bs/2, (A-5) 

where fcl2 = carrier frequency of 
group 1, 

and fc34 = carrier frequency of 
group 2. 

Substituting equations A-3 and A-4 into 
A-5 and solving for the carrier of the 
first group yields 

fcl2 = fc34/3 + 6,000 Hz. (A-6) 

The closest the two groups could be is 
the point where the highest frequency in 
the bandstop of the first group is equal 
to the lowest frequency in the bandpass 
of the second group, or 

fcl2 + bs/2 = fc34 - bp/2. (A-7) 

Then, from A-3 and A-4, 

fc34 = fcl2 + 10.8 kHz. (A-8) 

The simultaneous solutions to A-6 and A-8 
yield 

fcl2 = 14.4 kHz (A-9) 

and fc34 = 25.2 kHz. (A-10) 

Then, from A-2, 

f , = fcl2 - df/2 = 14.4 kHz 

- 3,600 Hz/2 = 12.6 kHz, (A-U) 
f2 = fcl2 + df/2 = 14.4 kHz 

+ 3,600 Hz/2 = 16.2 kHz, (A-12) 
fj = fc34 - df/2 = 25.2 kHz 

- 3,600 Hz/2 = 23.4 kHz, (A-13) 



and 



f4 = fc34 + df/2 = 25.2 kHz 

+ 3,600 Hz/2 = 27.0 kHz. (A-14) 

The application note AN-025 contains a 
worked-out example for the design of the 
filter shown in figure A-1. An addition- 
al reference may be necessary for better 
understanding of filters as used in oper- 
ational amplifiers. In particular, an- 
other reference^ was consulted for better 
definition of the individual component 
calculations. All of the capacitors of 
figure A-1 can be set equal, and the re- 
sistor values become 



Rlx 



Qx 



Ho 2Trfx C 



R2x = 



R3x = 



Qx 



(2Qx - Ho) 2Trfx C 
2Qx 



2Trfx C 



where Qx = the Q (ratio of reactance to 
resistance) of the individ- 
ual stages A, B, and C, 

fx = the center frequency 
of the stage. 

Ho = the gain of the stage, 

and C = the value of capacitance 
chosen. 
Either of the references (cited in 
footnotes 2 and 6) can be consulted for 
the procedures to determine the Q and 
center frequencies of the stages. The 
gain is a free parameter and can be cho- 
sen for the desired amplitude response. 
In practice, Rl is much greater than R2; 



-"Work cited in footnote 2. 

^Graeme, J. G. , G. E. Tobey, and L. P. 
Huelsman. Active Filters. Ch. 8 in Op- 
erational Amplifiers, Design and Applica- 
tions. McGraw-Hill, 1971, p. 293. 



16 



so R2 can be used to trim the Q. Then, 
to adjust the center frequency, R2 and R3 
can be simultaneously adjusted by the 
same percentage with negligible effect on 
the Q. 

Figure A-2 is a set of reproduced pic- 
tures from a spectrum analyzer, showing 
the response of the filter used in the 
LSM. This filter is centered at 25.2 
kHz, the carrier of the RCCM, and has an 
overall Q of 3.5 with unity gain in the 
bandpass. Amplitude is given in decibels 
with a reference level of 1 V root mean 
square (RMS). 

Panel 1 , of figure A-2 shows a noise 
source used as an input to test the indi- 
vidual stages of the filter. Panel 2 
shows the output of stage A with the 
above noise at the input. This stage has 



a Q of 14 and a gain of 12 dB (Ho = 4) at 
its peak frequency. Panel 3 shows the 
output of stage B with noise at the in- 
put. It is a reflection of stage A, 
about the geometric center of the filter, 
with the same characteristics. Panel 4 
shows the output of B with noise at the 
input of A and the output of A into B. 
Panel 5 shows the output of stage C with 
noise at the input. This stage has a Q 
of 7 and a gain of 12 dB at 24.94 kHz, 
the geometric center of the 3-dB bandpass 
points. Panel 6 shows the final output 
of stage C with noise at the input of A, 
and Panel 7 shows a comparison of input 
to output. 

Figure A-3 is a reproduced picture of 
the spectrum response from an RCCM. The 




-40 

-10 

-20 

-30 

-40 

-30 

-40 

-50 

-60 



. .y^ ^'''''''''^. . - 



28.75 




25.25 



-10 




+2 +4 +6 -t-8 +10 



-6 -4 -2 +2 +4 +6 +8 +10 -|0 -i 

FREQUENCY, kHz 

FIGURE A-2. - Filter response. ], Noise source input; ~, stage A; 3, stage B; J, stages A 
combined; .5, stage C; 6, stages A, B, and C combined; 7, input-to-output comparison. (Number 
each panel is the center frequency of the panel, in kilohertz.) 



and B 
below 



17 



top waveform was measured at TP4 of the 
LSM (fig. 6) and shows the unfiltered re- 
sponse. The two peaks correspond to mark 



and space frequencies transmitted from 
the RCCM. The bottom trace was measured 
at TP5 and shows the filtered response. 



-70 



TP4 raw 




15 17 19 21 23 25 27 

FREQUENCY, kHz 

FIGURE A-3. - RCCM filtered response. 



29 



33 



35 



18 



APPENDIX B. — MODULATION DESIGN AND CALIBRATION 



The modulator and demodulator used for 
the LSM and RCCM IG's are the XR2206 and 
XR2211, respectively, from Exar Integrat- 
ed Systems, Inc. The application notes^ 
on these circuits cover the theory of op- 
eration. The justifications for the 
equations as well as worked-out examples 
are included in those notes. 

The design solutions for the RCCM and 
LSM demodulators are presented first. 
Refer to figure B-1. 



Choose the value of the timing resistor 
(RO) between 10 and 100 kQ or -50 kf2. 

Then CO = 1/(R0 * fo) = 0.00139 pF; 

let CO = 0.001 uF, 5% for 
convenience, 

RO = 1/(C0 * fo) = 69.4 kQ 
= 64.9 k$7, 1% 

+ 10 kO, potentiometer. 



RCCM DEMODULATOR 

Given that f, = 12,600 Hz and £2 = 
16,200 Hz, calculate PLL center frequency 
as fo = (f , + f2)/2 = 14,400 Hz. 



Exar Integrated Systems, Inc. (Sunny- 



vale, CA). Function Generator Data Book. 
May 1981, 53 pp.; Phase-Locked Loop Data 
Book. Feb. 1981, 85 pp. 



Calculate Rl to give a lock range equal 
to the mark-space deviation as 

Rl = (RO * fo)/(f2 - fl) 
= 277.7 kfl -270 kfl. 

Calculate CI to set loop damping equal 
to 0.5 as 



FSK 
input 



iCD 



VCO 
fine tune 




FIGURE B-1. - Demodular circuit. 



19 



CI = CO/4 = (0.001 uF)/4 

= 250 pF -270 pF. 

Calculate CF for data filer time con- 
stant as 

CF(uF) = 3/baud rate = 3/4,800 

= 625 pF -620 pF. 

The capture range, dfc, is recommended 
to be 80% to 95% of the lock range. 
Picking 90% yields dfc = 3,240 Hz. For 
the full-tracking bandwidth, the lock- 
detect filter capacitance is given as 
16/2 dfc (in microfarads). 

CD (yF) = 16/(2 * 3,240) = 0.0025 pF. 

LSM DEMODULATOR 

Given that fj = 23,400 Hz and f^ = 
27,000 Hz, calculate PLL center frequency 
as fo = (f3 + f4)/2 = 25,200 Hz. 



The calculated values follow, as for 
the RCCM demodulator: 

CO = 1/(R0 * fg) = 0.00079 yF. 

Let CO = 0.001 uF, 5% for 
convenience. 

Then RO = 1/(C0 * fg) = 39.7 kJ2 
= 34.8 k^, 1% + 10 ka 
potentiometer, 

Rl = (RO * fo)/(f2 - fl) 
= 277.7 kD. -270 kn, 

CI = CO/4 = (0.001 yF)/4 
= 250 pF -270 pF, 

CF = 3/baud rate = 3/4,800 
= 625 pF -620 pF, 

CD = 16/(2 * 3,240) 
= 0.0025 yF. 
Next, the design solutions for the RCCM 
and LSM modulators are presented. Refer 
to figure B-2. 




0>2V p-f| 

^y . Q<IV-'fo 

Keying Y ^ 

input -L 



X 



FIGURE B-2. - Modulator circuit. 



20 



RCCM MODULATOR 



RCCM CALIBRATION 



The RCCM must transmit the mark and 
space frequencies of the LSM demodulator, 
so fj = 23,400 Hz, f4 = 27,000 Hz, and fg 
= 25,200 Hz. 

Typical values of Rl and R2 are -50 kO. 
so CO is chosen from 

CO = 1/(R * fo) = 1/(50 kn * 25,200) 

= 794 pF -820 pF, 5%. 

Then Rl and R2 are determined from 
R = 1/(C0 * fo): 

Rl = 1/(820 pF * 23,400) 
= 52.1 kn = 46.4 kJ2, 1% 
+ 10 kfi potentiometer. 

R2 = 1/(820 pF * 27,000) 
= 45.2 kfi = 40.2 kQ, 1% 
+ 10 k^ potentiometer. 

LSM MODULATOR 

The LSM must transmit the mark and 
space frequencies of the RCCM demodula- 
tor, so f , = 12,600 Hz, f2 = 16,200 Hz, 
and fo = 14,400 Hz. 

As in the RCCM modulator, 

CO = 1/(50 kfi * 14,400) 

= 1,389 pF -1500 pF, 5%, 

Rl = 1/(1,500 pF * 12,600) = 52.9 kO. 
= 47.5 kn, 1% + 10 kQ 
potentiometer, 

R2 = 1/(1,500 pF * 16,200) = 41.2 kn 
= 36.5 kfi, 1% + 10 kj^ 
potentiometer. 

Since the operation of this modem net- 
work requires only one LSM, the con- 
struction and calibration processes were 
performed at the engineer's level. 
Approximately 20 RCCM's were anticipated 
for the Lucerne No. 8 monitoring system; 
therefore, the construction and calibr- 
ation process was clearly defined such 
that the circuits could be built at the 
technical level. 



To aid in calibration of the RCCM, the 
circuit of figure B-3 was developed and 
is referred to as the RCCM calibrator. 
This circuit is adjusted to transmit the 
f 1 and f2 frequencies of the LSM and has 
connectors that directly mate with the 
RCCM. An XR2207 function generator is 
used to modulate the XR2206 with a 1-kHz 
and a 2.4-kHz square wave. The 1 kHz 
serves as a convenient scale when check- 
ing demodulated data symmetry on an os- 
cilloscope, and the 2.4 kHz checks the 
demodulator's performance at an equiva- 
lent 4,800-Bd rate. 

The calibration procedure sets the fre- 
quencies and power levels of the RCCM. 
Although the RCCM may be calibrated imme- 
diately after assembly, it is recommended 
that a final calibration be performed 
after a 168-h burn-in period. 

Test equipment 

• Oscilloscope, single trace 

• RMS voltmeter, 600 Q 

• Frequency counter 

• RCCM calibrator 

Procedure 

1. Connect plugs from calibrator to 
RCCM and power outlet. Set SI to f^ pos- 
ition and S2 to B,. The CD and gate ind- 
icators should be on. 

2. Connect RMS voltmeter and frequency 
counter in parallel. Attach their com- 
mon, along with the scope common, to 
TP7. 

3. Connect voltmeter-counter probe to 
TPl. The voltmeter should read about 
-10 dB. 

4. Move the voltmeter-counter probe 
to TP2. The voltmeter should read about 
-6 dB. 

5. Connect scope probe to TP3. Set 
vertical scale to 5 V per division and 
the horizontal scale to 0.5 ms , and syn- 
chronize the trace if possible. 

6. Adjust balance control for equal 
positive and negative durations. Depress 
S2 to B2 position. The scope trace 
should remain with equal positive and 
negative durations at a higher frequency 
(a small amount of jitter may be noted). 

7. Connect voltmeter-counter to TP5. 



21 




r 

o.ovfT 



C5 



RI4 
82 



RI5 
25 

kxi 




/J 



J // 

D 

10 

6 8 

9 12 






Rll 
4.7 kfl 



RI2 
3.9kn ^ 











RI8^ 

ka ' 


> 
> 

> 


J1 -2 
J1-4 


^si, 


1 




ii 


J1-1 








Ji -3 



. ^ 25 ^ 

B, '^^ 



cw 



C6 
I/.F 



S2 



t — v^AA- 

RI3 
ii 4.7 kil 



B' 



I 



15-V 
power 
supply 



i- C7 
lO/xF 



ac 
power 



FIGURE B-3. - RCCM calibrator schematic. 



22 



8. Adjust level for dB on 
voltmeter. 

9. Adjust £4 for 27.00 kHz. 

10. Move voltmeter-counter probe to 
TP6. There should be no output (less 
than -50 dB). 

11. Switch SI to fj position. The 
gate indicator should go out. 



12. Readjust level for dB on volt- 
meter (you should have to increase it by 
about 5 dB). 

13. Adjust fj for 23.40 kHz. 

14. Calibration is complete. Remove 
test equipment and seal RCCM adjustments. 



<^^ 




FIGURE B-4, - LSM printed circuit layout. 



23 



LSM CALIBRATION 

The modulator transmit frequencies of 
the LSM are adjusted while controlling 
the digital input at J2 pin 2 (see figure 
6). 2 The output level, as measured at 
TP3, is adjusted for dB with a 600-i^ 
resistor connected across TBl-4 and TBl- 
5 The reference oscillator is adjusted 
while monitoring at TP9 and the level set 

to -50 dB. . . 

To calibrate the demodulator, it is 
best to have a freshly calibrated RCCM. 
Modulate the RCCM, at TP4, with a 1-kHz 
square wave and connect its output to the 
input of the LSM. Adjust the demodulator 
for symmetry as measured at TP6. 

Figure B-4 is the printed circuit lay- 
out of the LSM shown in figure 6. Figure 
B-5 is the layout for the LSM line split- 
ter of figure 7. And lastly, figure B-6 
is the layout of the RCCM of figure 8. 




2Figure numbers without a B- prefix re- 
fer to figures in the main text. 



FIGURE B-5. - Line splitterprintedcircuit layout. 




FIGURE B-6. - RCCM printed circuit layout. 



INT.-BU.OF MINES, PGH., PA. 27962 



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